Most of the biological of the steroid hormone 1,25(OH)2 vitamin D3 are mediated through its interaction with the nuclear vitamin D receptor (VDR). VDR is a member of a superfamily of nuclear receptors which are ligand-dependent transcription factors. Other members of the family include receptors for steroid hormones, retinoids, thyroid hormone and several orphan receptors. Ligand binding induces conformational changes in the VDR that enable the receptor to form heterodimers with RXR, interact with other co-activators, and modulate gene transcription 1,25(oh)2D3, together with parathyroid hormone and calcitonin, is involved in the regulation of calcium homeostasis. Maintaining proper blood calcium levels is important for normal bone growth and maintenance. 1,25(OH)2D3 is also involved in the biological processes related to several diseases including osteoporosis, proliferative skin disorders like psoriasis, and some cancers including certain leukemias, breast and prostate cancers. The first steps in 1,25(OH)2D3 modulation of gene transcription involve binding of the 1,25(OH)2D3 ligand to VDR which results in conformational changes in the ligand binding domain (LBD) of VDR. We propose to investigate the molecular details of ligand binding and the relationship of binding to conformational changes in LBD. We propose the following specific aims to VDR that are important for binding 1,25(OH)2D3. 2) Determine if the VDR residues important for binding conformational changes in LBD of variant VDRs associated with binding 1,25(OH)2D3 or analogs of 1,25(oh)2D3. Together these experiments should help define the role of 1,25(OH_2D3 binding in the vitamin D signal transduction pathway and could lead to the development of more effective 1,25(oh)2D3 analogs for the treatment of particular diseases involving the vitamin D endocrine system.